FIG. 1 is a highly schematic block diagram, indicated generally by the reference numeral 1, of a known motor system. The motor system 1 comprises an AC power source 2, a rectifier 4, a DC link capacitor 6, an inverter module 8, a three-phase motor 10 and a controller 12.
As is well known in the art, the rectifier 4 converts AC electrical power provided by the AC power source 2 into a DC source at the DC link capacitor 6. The inverter module 8 comprises a number of switching elements, typically insulated gate bipolar transistors (IGBTs), that are used to convert the DC signal at the DC link capacitor 6 into three AC signals that are provided to each of the phases of the motor 10. The controller 12 provides switching instructions for each of the switching elements of the inverter module 8. Thus, the controller 12 is able to precisely control the frequency and phase of each of the signals provided to the motor 10.
The controller 12 may, for example, be used to control the motor 10 in order to provide a desired speed and/or torque. In order to enable accurate control, it is necessary for the controller 12 to take into account the electromagnetic properties of the motor 10.
One method is to use data sheet information relating to the motor 10. However, even when this information is available, it is often insufficiently precise and accurate to enable accurate and efficient control of the motor 10.
An alternative to using data sheet information is to measure the characteristics of the motor itself. For example, it is known to use the controller 12 to control the injection of signals into the motor 10, to monitor the response to those signals and to estimate various resistances and inductances of the motor 10 on the basis of those responses.
In some cases, it is desirable to inject large currents into the motor to mitigate nonlinearities caused by the inverter or to explore other nonlinearities such as magnetic saturation. Injecting large currents into a motor can cause significant heat generation and can cause damage to the motor and/or the inverter. Further, some existing methods for obtaining data regarding the characteristics of the motor 10 are slow.
Many existing methods require the motor 10 to rotate in order to determine the electrical and magnetic properties of the motor. With the motor 10 installed within a system, this may often be undesirable. It would therefore be advantageous in some circumstances to enable such data to be obtained with the motor at standstill.
The present invention seeks to address at least some of the problems outlined above.